The present invention relates to thin film heterojunction photovoltaic cells having copper indium diselenide (CIS) as an active semiconductor layer and more particularly to a method for forming such CIS films.
References which illustrate the background of photovoltaic devices including CIS semiconductor layers include U.S. Pat. NO. 4,335,266 issued to Mickelsen et al on June 15, 1982 and U.S. Pat. No. 4,465,575 issued to Love et al on Aug. 14, 1984. Both of these patents are hereby incorporated by reference. The Miceelsen patent provides considerable background information concerning development of CIS/cadimum sulfide (CdS) photovoltaic cells. Mickelsen teaches an improvement in the CIS deposition process involving the deposition of the CIS film in two sligthly different regions. The first region has an excess of copper while the second region, which is ultimately adjacent the CdS layer, is copper deficient. Diffusion between the two regions forms the desired CIS layer while reducing the probability of formation of pure copper nodules at the device junction. The CIS materials are deposited in Mickelsen using the reactive evaporation technique in which the three elements are simultaneously evaporated onto a heated substrate to form the compound semiconductor as deposited.
The Love et al patent teaches a different reactive deposition method for manufacturing the same type of device as Mickelsen et al. The primary difference is the use of DC magnetron sputtering devices to deposit the copper, indium and selenium. In a preferred form, Love uses Cu.sub.2 Se and In.sub.2 Se .sub.3 targets to deposit CIS films. Alternatively, Love teaches codeposition of elemental copper, indium and selenium in a reactive deposition process.
Various other techniques have also been used to deposit CIS films. For example, the publication "RF-Sputtered CuInSe.sub.2 Thin Films" by J. Piekoszewski et al, Solar Energy Materials 2(1980) 363-372, teaches the deposition of CIS films by RF sputtering from a synthesized target onto a heated substrate. Piekoszewski teaches that the particle size of powder from which the target is pressed is critical in terms of final film quality. Thus when a fine powder was used the resulting films were selenium deficient and indium rich. In the publication "Large Grain Copper Indium Diselenide Films" by T. L. Chu et al, J. Electrochem. Soc., September 1984, page 2182, two other film deposition techniques were discussed. These techniques involve either vacuum evaporation or electroplating of separate copper and indium films and then heat treatment of the resulting compound film in an atmosphere containing selenium. This publication notes that the control of the copper to indium ratio is essential and use of separate deposition steps enhances the ability to control the ratio of materials deposited.
Two other publications discuss other methods for depositing CIS films and teach photovoltaic structures in which a junction is formed between a CIS film and a zinc oxide film. The first of these is entitled "A ZnO/p-CuInSe.sub.2 Thin Film Solar Cell Prepared Entirely by Spray Pyrolysis" by M. S. Tomar et al, Thin Solid Films, 90 (1982) 419-423. In Tomar, zinc oxide was deposited on a tin oxide coated glass substrate by spray pyrolysis at a substrate temperature of from 350.degree. to 550.degree. C. After similar deposition of a CIS layer, photovoltaic response of about 2% efficiency was measured. In the report entitled "Chemical Vapor Deposited Copper Indium Diselenide Thin Film Materials Research", Final Report, March 1984, SERI/STR-211-2247 by Poly Solar, Incorporated, zinc oxide films were ion beam sputtered onto CIS films which were deposited by a close space chemical vapor transport technique. Photovoltaic efficiencies in the range of 2 to 3% were achieved by such devices.
The above-referenced patents and publications indicate that much effort has been made to develop practical techniques for depositing high quality CIS films for use in making good photovoltaic devices. While reasonable efficiencies have been achieved for very small area research type devices, no good method has been found for manufacturing large area, for example, at least one foot by one foot, devices. As noted in the various publications, it is essential that the proper stoichiometric ratio of materials be deposited to form the film. This has proven to be difficult even in research devices. For the large commercial devices very good uniformity must be achieved across large surface areas on a repeatable basis.